1 files changed, 36 insertions, 45 deletions

diff --git a/src/lib/libcrypto/ec/ec_asn1.c b/src/lib/libcrypto/ec/ec_asn1.c
index acab513748..8d0f032907 100644
--- a/src/lib/libcrypto/ec/ec_asn1.c
+++ b/src/lib/libcrypto/ec/ec_asn1.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ec_asn1.c,v 1.70 2024/10/14 12:42:52 tb Exp $ */
+/* $OpenBSD: ec_asn1.c,v 1.71 2024/10/14 12:50:18 tb Exp $ */
 /*
  * Written by Nils Larsch for the OpenSSL project.
  */
@@ -602,13 +602,39 @@ ec_asn1_group2fieldid(const EC_GROUP *group, X9_62_FIELDID *field)
 }
 
 static int
+ec_asn1_encode_field_element(const EC_GROUP *group, const BIGNUM *bn,
+    ASN1_OCTET_STRING *os)
+{
+        unsigned char *buf;
+        int len;
+        int ret = 0;
+
+        /* Zero-pad field element per SEC 1, section 2.3.5. */
+        len = (EC_GROUP_get_degree(group) + 7) / 8;
+
+        /* One extra byte for historic NUL termination of ASN1_STRINGs. */
+        if ((buf = calloc(1, len + 1)) == NULL)
+                goto err;
+
+        if (BN_bn2binpad(bn, buf, len) != len)
+                goto err;
+
+        ASN1_STRING_set0(os, buf, len);
+        buf = NULL;
+        len = 0;
+
+        ret = 1;
+
+ err:
+        freezero(buf, len);
+
+        return ret;
+}
+
+static int
 ec_asn1_group2curve(const EC_GROUP *group, X9_62_CURVE *curve)
 {
         BIGNUM *a = NULL, *b = NULL;
-        unsigned char *buffer_1 = NULL, *buffer_2 = NULL, *a_buf = NULL,
-        *b_buf = NULL;
-        size_t len_1, len_2;
-        unsigned char char_zero = 0;
         int ret = 0;
 
         if (!group || !curve || !curve->a || !curve->b)
@@ -619,50 +645,17 @@ ec_asn1_group2curve(const EC_GROUP *group, X9_62_CURVE *curve)
                 goto err;
         }
 
-        /* get a and b */
         if (!EC_GROUP_get_curve(group, NULL, a, b, NULL)) {
                 ECerror(ERR_R_EC_LIB);
                 goto err;
         }
-        len_1 = (size_t) BN_num_bytes(a);
-        len_2 = (size_t) BN_num_bytes(b);
-
-        if (len_1 == 0) {
-                /* len_1 == 0 => a == 0 */
-                a_buf = &char_zero;
-                len_1 = 1;
-        } else {
-                if ((buffer_1 = malloc(len_1)) == NULL) {
-                        ECerror(ERR_R_MALLOC_FAILURE);
-                        goto err;
-                }
-                if ((len_1 = BN_bn2bin(a, buffer_1)) == 0) {
-                        ECerror(ERR_R_BN_LIB);
-                        goto err;
-                }
-                a_buf = buffer_1;
-        }
 
-        if (len_2 == 0) {
-                /* len_2 == 0 => b == 0 */
-                b_buf = &char_zero;
-                len_2 = 1;
-        } else {
-                if ((buffer_2 = malloc(len_2)) == NULL) {
-                        ECerror(ERR_R_MALLOC_FAILURE);
-                        goto err;
-                }
-                if ((len_2 = BN_bn2bin(b, buffer_2)) == 0) {
-                        ECerror(ERR_R_BN_LIB);
-                        goto err;
-                }
-                b_buf = buffer_2;
+        if (!ec_asn1_encode_field_element(group, a, curve->a)) {
+                ECerror(ERR_R_EC_LIB);
+                goto err;
         }
-
-        /* set a and b */
-        if (!ASN1_STRING_set(curve->a, a_buf, len_1) ||
-            !ASN1_STRING_set(curve->b, b_buf, len_2)) {
-                ECerror(ERR_R_ASN1_LIB);
+        if (!ec_asn1_encode_field_element(group, b, curve->b)) {
+                ECerror(ERR_R_EC_LIB);
                 goto err;
         }
 
@@ -688,8 +681,6 @@ ec_asn1_group2curve(const EC_GROUP *group, X9_62_CURVE *curve)
         ret = 1;
 
  err:
-        free(buffer_1);
-        free(buffer_2);
         BN_free(a);
         BN_free(b);